Aldehydes are commonly prepared and hydrogenated into a corresponding alcohol. A difficulty associated with the process is the oxidation of the aldehyde to form a carboxylic acid by-product. The presence of carboxylic acid, especially if left unneutralized, may have a negative effect on the performance of most heterogeneous hydrogenation catalysts. Additionally, the carboxylic acid may react with the alcohol formed during hydrogenation resulting in additional yield losses and additional separation costs. The carboxylic acid may cause corrosion of processing equipment, especially when present in process streams heated above ambient temperature. Typically, the carboxylic acid is partially neutralized prior to hydrogenation. For example, U.S. 2004/0087819 discloses neutralization of an aqueous 3-hydroxypropionaldehyde solution prior to hydrogenation. However, partial neutralization through the addition of a base, typically an alkali base, is problematic due to the potential degradation of the aldehyde resulting from inefficient mixing. The aldehyde, in the presence of excess base occurring from inefficient mixing, can combine to form byproducts such as acetals and/or aldols which can undergo further condensation to yield polymeric heavy ends. Some of the acids formed are known to be hydroxyacids, where neutralization alone may not fully eliminate the negative impact on the hydrogenation catalyst. Additionally, the resulting alkali metal salt formed during partial neutralization imparts an ash component which substantially reduces the market value of the heavy ends co-product, and the alkali metal salt formed can foul equipment downstream such as the reboilers of downstream distillation columns and heat exchangers.
1,3-propanediol is an industrially important chemical. 1,3-propanediol is used as a monomer unit to form polymers such as poly (trimethylene terphthalate) that are used in the production of carpets and textiles. 1,3-propanediol is also useful as an engine coolant, particularly in cooling systems that require coolants having low conductivity and low corrosivity.
1,3-propanediol may be prepared in a two-step process in which ethylene oxide is first hydroformylated in an organic solution in the presence of a metal catalyst such as a cobalt or rhodium carbonyl to form 3-hydroxypropionaldehyde. The 3-hydroxypropionaldehyde intermediate is water extracted from the organic phase under pressure and the metal catalyst is recycled to the hydroformylation reaction in the organic phase. In a subsequent step, the aqueous 3-hydroxypropionaldehyde is hydrogenated to 1,3-propanediol.
Ideally, the aqueous 3-hydroxypropionaldehyde extract could be routed directly to the hydrogenation reactor. However, as discussed above, the carboxylic acid formed as a byproduct during hydroformylation may have a negative effect on the performance of most heterogeneous hydrogenation catalysts. Additionally, the small amount of metal from the hydroformylation catalyst that typically leaches into the water phase during extraction of 3-hydroxypropionaldehyde also may have a negative effect on the performance of most heterogeneous hydrogenation catalysts.
U.S. 2004/0087819 discloses removing a hydroformylation catalyst from an aqueous 3-hydroxypropionaldehyde solution by utilizing a cation exchange resin. As discussed hereinbefore, the reference also discloses neutralization of the aqueous 3-hydroxypropionaldehyde solution. The neutralization occurs after contact with the cation exchange resin and before hydrogenation.
It goes without saying that it is highly desirable to improve the process for preparing an alcohol from an aldehyde.